I'm not a car person so I'm slowly learning how to deal with them but this problem happens so randomly and has happened twice.

98 Subaru Outback Legacy, Automatic

Problem:
The issue is that when driving, very randomly and only for about 1 mile if that, the tranmission seems to lurch. The car sort of shudders and the RPMs increase dramaticly when trying to get the car rolling. It feels like the tranmission just sticks in first gear or something. But it happens randomly and the tranny fluid is beautiful in color and full.